The infusion of donor lymphocytes (DLI) induces remission in approximately 75 percent of patients with relapsed chronic myelogenous leukemia (CML) after allogeneic bone marrow transplantation. The efficacy of DLI reveals that immune responses against leukemia cells can be mounted in the absence of complicating chemotherapy or radiation treatment, and is thus a unique model for studying tumor immunity. How DLI mediates this anti-leukemic effect is unclear. T cells are required for this response, but their antigenic targets are unknown. Recent reports have demonstrated the presence of autoantibodies reactive to various tumors and have utilized autologous sera to identify novel tumor-associated antigens, which were later shown to be recognized by T cells as well (for review, see Old et al., 1998). Our preliminary results reveal that an analogous approach may succeed in identifying CML-associated antigens in patients who have responded to DLI. In our trial which used CD4+-DLI to treat relapsed CML, many patients were observed to develop marked elevations in their peripheral B cell number and frequent lymphocytosis and plasmacytosis on bone marrow biopsies, suggesting the presence of a potent B cell response. Immunoscreening of a CML cDNA expression library with high titer post-DLI sera identified eight gene products with antibody reactivity to patient sera collected after but not before DLI. Herein, we propose to characterize the humoral and cellular response to these DLI-associated targets. The CML cDNA expression library will be further screened with other patients' post-DLI sera in order to expand the panel of DLI-associated antigens. Our questions will focus on determining whether the identified antigens are shared among DLI-treated patients (in effort to understand whether leukemia-specific targets or host-donor differences explain the basis of anti-tumor immunity), on delineating the time course of antibody response; and on ascertaining whether the presence of antibody reactivity correlates with a clinical response to DLI, or to the development of graft-versus-host disease (GVHD). To gain insight into whether the identified genes play a role in leukemogenesis, the genes will be fully sequenced, and differential expression between normal and malignant cells will be examined by Northern analysis. Lastly, because T cells are critical to DLI, the gene products will be tested for their ability to induce an antigen- specific T cell response in CML patients' peripheral blood leukocytes in culture. The experiments described in this proposal will lead to a deeper understanding of the mechanisms underlying the antitleukemic effect of DLI and provide a basis for improved diagnosis and treatment of CML and potentially other hematological malignancies.